1. Field of the Invention
The present invention relates to an image output system, and more particularly, to a dithering method and apparatus of the image output system, capable of displaying an image without reducing the number of gray levels of input data of a high gray level image.
2. Description of Related Art
An image output system is developing as various devices such as a cathode-ray tube (CRT), a liquid crystal display (LCD), a plasma display panel (PDP) and a mobile display. A typical method for outputting an image may include converting a practical image to a digitalized signal, performing image processing on the digitalized signal, and displaying the processed video signal through the image output system. In the above processing sequence, the image output system should output an image closest to the practical image. That is, data to be lost in the process of digitalizing the practical image should be minimized and an amount of lost data of the image-processed image should be minimized. The process of digitalizing the practical image includes a sequence of processes such as sampling, quantization and normalization. One object of the sequence of processing signals is to minimize data to be lost so that digital data are closest to the practical image.
The image output system is an apparatus for displaying the processed image to be visible to the naked eye, but it has limitations. That is, the image output system has a limitation in the number of gray levels that it can express. For instance, when each of R, G and B video signals consists of 8 bits, one video signal can express 28 numbers of gray levels. By synthesizing the R, G and B video signals, it is possible to express 28×28×28 numbers of colors, i.e., 224 numbers of colors. However, if the image output system outputs an 8-bit video signal as a 6-bit signal, each video signal cannot express (28−26) numbers of gray levels and thus it cannot express (224−28) numbers of colors. Therefore, the image output system expressing the number of gray levels that is smaller than that of an original video signal employs dithering technology to implement an image closest to the practical image.
Each of pixels constructing one image includes 3 sub-pixels consisting of R, G and B. Each of the sub-pixels is provided with a video signal. If the number of gray levels of the video signal coupled to each sub-pixel is reduced, a false contour line generating an obvious contour line at a boundary of a screen may be generated or a Mach's phenomenon of generating a bright or dark band on the screen may occur.
Since the false contour line or the Mach's phenomenon generates the obvious contour line that does not exist in the practical image, it becomes a cause of deteriorating image quality. Thus, in order that the false contour line or the Mach's phenomenon is not generated, the dithering is performed to smoothly process the obvious contour line by intentionally inputting noises to data or pixels at a boundary of the image. In general, in case a bit width of a video source is greater than that of the image output system, the following two schemes may be used.
The first one is a truncation scheme.
The truncation scheme is technology of simply removing lower 2 bits of a video signal coupled to a pixel. For instance, in case the video signal has 8 bits, 6 bits except the lower 2 bits are outputted as an output signal. When constructing a screen by inputting the signal of 6 bits to the pixel, since the number of gray levels of one sub-pixel becomes 26, the boundary of the image may be definitely outstood.
FIG. 1 shows a truth table representing the truncation scheme.
Referring to FIG. 1, in case input data has 8 bits, since decimal numbers 0, 1, 2 and 3 are outputted as 0 without discrimination in a process of expressing the input data with 6 bits, the image displayed through the image output system may have a false contour line unlike the practical image.
The second one is a temporal/spatial compensation scheme.
The temporal/spatial compensation scheme is technology of applying a spatial effect of reflecting lower 2 bits onto a pixel and a line by determining positions of the pixel and the line to be compensated and a temporal effect of reflecting the lower 2 bits to each frame, with reference to the lower 2 bits to be discarded in case input data has 8 bits and output data has 6 bits. That is, the temporal/spatial compensation scheme is a scheme of expressing the output data of 6 bits closer to 8 bits. The reflection of the lower 2 bits is to compensate the lower 2 bits that become a weight to a line and a pixel positioned in each frame.
Table 1 represents the temporal/spatial compensation scheme according to the lower 2 bits.
TABLE 1Lower 2 bits1st frame2nd frame3rd frame4th frame000000010+10010+10+1011+10+1+1
As described in Table. 1, for one pixel of each of the first to fourth frames, a weight 1(100) is added to higher 6 bits except the lower 2 bits among bits of the input data or the higher 6 bits are outputted just the same according to a value of the lower 2 bits.
If the lower 2 bits to be discarded have a value of ‘11’ and the value is maintained for the 4 frames, the output data loses a value, i.e., 3 (value of lower 2 bits, 11)×4 (no. of frames)=12. A method for compensating the lost value is to add 1(100) to higher 6 bits of corresponding pixels of the first, third and fourth frames and to output higher 6 bits of a corresponding pixel of the second frame just the same. If the compensation is completed, a value, 4(100)×3 (no. of frames where 1 is added)=12, is compensated and thus the compensated value always becomes equal to the value lost during the 4 frames.
For one more example, in case the lower 2 bits to be discarded have a value ‘10’, the output data loses a value, i.e., 2 (value of lower 2 bits, 10)×4 (no. of frames)=8. A method for compensating the lost value is to add 1(100) to higher 6 bits of corresponding pixels of the first and third frames and to output higher 6 bits of corresponding pixels of the second and fourth frames just the same. If the compensation is completed, a value, 4(100)×2 (no. of frames where 1 is added)=8, is compensated and thus the compensated value always becomes equal to the value lost during the 4 frames.
There is no limitation in the position of a frame where the weight 1 is added in the temporal/spatial compensation scheme. For instance, in case the lower 2 bits to be discarded have the value of ‘11’, the weight is applied to pixels of 3 frames among continuous 4 frames. In case the lower 2 bits to be discarded have the value of ‘10’, the weight is applied to pixels of 2 frames among the continuous 4 frames.
However, the temporal/spatial compensation scheme according to the prior art has the following problems.
FIG. 2 provides a truth table representing the conventional temporal/spatial compensation scheme in case an input video signal has 8 bits. FIG. 3 illustrates a graph showing output performance when an output signal is normalized in a range of 0 to 100 according to the conventional temporal/spatial compensation scheme.
As shown in FIG. 2, the overflow may occur when performing the temporal compensation for a gray level greater than a decimal number 252 among the 8 bits of the input video signal. Therefore, although the temporal/spatial compensation scheme is applied, the compensation cannot be implemented. In this case, as shown in FIG. 3, the gray level saturation may occur in higher gray levels of input data regardless of the input variation. Therefore, when performing the dithering by applying the conventional temporal/spatial compensation scheme, there may be caused a problem of not expressing high luminance parts stably.